1. Field of the Invention
The invention relates to a display device constituted including a transistor and a driving method of the display device. In particular, the invention relates to a semiconductor device including a pixel constituted including a thin film transistor (hereinafter also called a transistor).
2. Description of the Related Art
An active matrix display, which is constituted by the combination of an electroluminescence element (also called an organic light emitting diode (OLED) and an EL element or a light emitting element in this specification) and a transistor, has been attracting attentions and actively researched and developed both domestically and internationally as a thin and lightweight display. This display which is also called an organic EL display (OELD) is extensively researched and developed in a practical use stage aiming for a small 2-inch display to a large display of 40-inch or larger.
Luminance of an EL element and a current value flowing therethrough are theoretically in a linear relationship. Therefore, for an organic EL display which employs an EL element as a display medium, a method to express a gray scale by controlling a current value supplied to the EL element is known. Moreover, as a method to control a current value supplied to the EL element, a voltage input driving method and a current input driving method are known.
In the voltage input driving method, a current value supplied to a driving transistor (hereinafter also called a driving transistor) and an EL element is controlled by a gate-source voltage obtained by inputting a voltage signal to a gate of a driving transistor so as to be held therein, which is connected in series to the EL element. In the current input driving method, a current value supplied to a driving transistor and an EL element is controlled by a gate-source voltage of a driving transistor obtained by supplying a current signal to the driving transistor (for example, refer to Patent Document 1).
However, in a conventional current input driving method, a slight amount of current is required to be supplied from a source signal line to express a low gray scale. As time to charge parasitic capacitance of a source signal line or the like is required to input a slight amount of current as a video signal to a pixel, there is a problem in that long writing time is required.
Further, as another example of a current input driving method, such a pixel is known, in which by holding Vgs inputted as a current to a driving TFT and a threshold voltage thereof in two capacitors and capacitively coupling them, a current supplied to an EL element can be smaller than an actual video signal while compensating the threshold voltage (for example, refer to Patent Document 2).
However, even such a pixel configuration requires a period T1 to obtain a threshold voltage and a period T2 to write a video signal. As an area of one pixel is limited, the capacitance of two capacitors are also limited. Therefore, there is a problem in that there is not enough writing time for writing a slight amount of current as a video signal, and in a large panel in particular, a writing period per pixel becomes shorter as compared to a small panel.    [Patent Document 1]    International Publication No. 9848403    [Patent Document 1]    Japanese Patent Laid-Open No. 2004-310006